In general, a plastic rod lens is a lens element having a cylindrical shape and is constituted of plastic materials. Its refractive index decreases continuously from its central axis to its outer peripheral portion to form an erect image of the same size. The plastic rod lens is widely used in the form of a rod lens array where both end surfaces of each lens are mirror polished and many of these polished lenses are arranged closely and then bonded/integrated. The application includes components of an image sensor in various scanning devices (e.g. a hand scanner), a copier, and a facsimile; and a write device of an LED printer. Since the plastic rod lens can be manufactured at low cost and the optical system installed with the lens can be made compact, its field of application is expanding.
Particularly due to the rapid circulation of a printer combined with a color scanner with a high resolution of 600 dpi or higher in recent years, a demand for a plastic rod lens with favorable color characteristics (i.e. with a small chromatic aberration) is increasing. However, it has been difficult to obtain a plastic rod lens having a small chromatic aberration since, for example, only a limited choice of materials is available compared to the glass rod lens.
Patent Document 1 discloses that it is possible to obtain a plastic rod lens having a small chromatic aberration where the difference in Abbe numbers in the plastic rod lens is 2 or less by combining an alicyclic methacrylate with a methyl methacrylate (MMA). In addition, Patent Document 2 discloses that it is possible to obtain a plastic rod lens having a small chromatic aberration by using an alicyclic methacrylate, MMA, and a fluorinated alkyl methacrylate. In the techniques disclosed in the above documents, the Abbe number at the center of a plastic rod lens is equal to or smaller than the Abbe number in its outer peripheral portion, and although smaller chromatic aberrations compared to those of the conventional plastic rod lens are achieved, a plastic rod lens having a small chromatic aberration which is equivalent to that of a glass rod lens has not been obtained.
Additionally, Patent Document 3 discloses a gradient index optical element which is made from a synthetic resin composed of a copolymer using 3 kinds of monomers. With this optical system, a spectacle lens or a contact lens having a characteristic (i.e. the refractive index increases as the Abbe number increases) is achieved. However, since a bromine-containing monomer or a crosslinkable monomer which is corrosive to a manufacturing facility is used as a monomer having high refractive index, it was difficult to apply the technique to the manufacturing process of a plastic rod lens.
Further, Non-Patent Document 1 discloses the following formulae for expressing chromatic aberration.
                                          Δ            ⁢                                                  ⁢            P                    P                =                ⁢                              -                          1              2                                ⁢                                    Δ              ⁢                                                          ⁢              A                        A                                                  =                ⁢                              1            2                    ⁢                                                                                          N                    0                                    ·                  Δ                                ⁢                                                                  ⁢                                  N                                      r                    ⁢                                                                                  ⁢                    0                                                              -                                                                    N                                          r                      ⁢                                                                                          ⁢                      0                                                        ·                  Δ                                ⁢                                                                  ⁢                                  N                  0                                                                                    (                                                      N                    0                                    -                                      N                                          r                      ⁢                                                                                          ⁢                      0                                                                      )                            ·                              N                0                                                        (In the formula, P represents a period length, Δ represents changes due to the wavelength, A represents a positive constant, N0 represents a refractive index on the central axis, and Nr0 represents a refractive index at a point where the distance from the central axis is r0. Note that the relationship among P, A, and N0 is expressed by the following formulae.)
      P    =                  2        ⁢        π                    A                  N    =                  N        0            (              1        -                              A            ·                          r              2                                2                    )      (N: refractive index at a point where the distance from the central axis is r.)
However, there were not any Specific Descriptions of the Plastic Rod Lens with a small chromatic aberration.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 7-35929
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2000-35517
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 9-127310
Non-Patent Document 1: K. Nishizawa, Appl. Opt. 19, 1052 (1980)